Ultrafast Dynamics Group
Frédéric Laquai's Group


Organic Photovoltaics:​​
Impact of Fullerene.png

Impact of Fullerene on the Photophysics of Ternary Small Molecule Organic Solar Cells - Our recent work published in Advanced Energy Materials (Karuthedath et al., Adv. Energy Mater. 2019,1901443​) discusses the impact of fullerene on the photo physics of ternary small molecule non-fullerene acceptor (NFA) solar cells. We observed that after excitation of PC71BM, ultrafast singlet energy transfer to the NFA (ICC6) occurs. The singlet states on ICC6 then undergo hole transfer to DR3 (the small molecule donor), leading to free charge generation. Interestingly, PC71BM has a dual role: firstly, it increases the blend’s photoresponse in the UV spectral region and secondly it improves the electron mobility of the ternary blend, resulting in less recombination and faster electron extraction. Finally, we used the (photo)physical parameters obtained from (transient) spectroscopy and charge transport measurements to simulate the device's current–voltage characteristics and we found that the simulation accurately reproduces the experimentally-measured device JV-characteristics.

Charge and Triplet Exciton Generation in Neat PC70BM Films and Hybrid CuSCN:PC70BM Solar Cells - We recently investigated charge and triplet exciton dynamics in neat fullerene and fullerene-rich hybrid solar cells by means of transient spectroscopy (Karuthedath et al. Adv. Energy Mater., 2018, DOI: 10.1002/aenm.201802476). The efficiency and mechanism of charge generation and triplet formation depend on the photoactive layer composition. Formation of a hybrid bulk heterojunction leads to ultrafast exciton dissociation, fast charge extraction, and PCEs in excess of 5%. In neat PC70BM films, charge generation is slow and occurs with low yield, while the hybrid films show ultrafast generation due to the presence of abundant interfaces, facilitating charge separation. In PC70BM films, the triplet exciton generation is proportional to the product of singlet and charge concentrations, indicating a charge–singlet spin exchange mechanism. In hybrid films however, the triplet formation is a consequence of non-geminate recombination of charges. At low fluences, the fraction of charges outweighs the population of triplets in both, blends and neat films, leading to respectable device efficiencies under one sun illumination.​​


The​​rmal annealing reduces geminate recombination in TQ1:N2200 all-polymer solar cells – We studied the effect of thermal annealing on the photophysics and efficiency of a prototypic all-polymer bulk heterojunction, namely TQ1:N2200. Annealing doubles the external quantum efficiency (EQE) and improves the device fill factor (FF), resulting in an increased power conversion efficiency. A combination of transient spectroscopic experiments including TREFISH, TPC, and TAS revealed:  the increased EQE originates from differences in charge carrier separation and (geminate) recombination at the polymer–polymer interface. In as-spun samples 35% of the photo-generated charges are bound in interfacial charge-transfer (CT) states and recombine geminately, while this pool is reduced to 7% in thermally-annealed samples, resulting in higher short-circuit currents. We believe that changes in the nature of the donor/acceptor interface cause the difference in charge separation and recombination and show that the interface properties can be altered to enhance charge.       
separation without compromising charge transport and extraction. This work was published recently in (S. Karuthedath et al., J. Mater. Chem. A 2018, 6, 7428).​

Impact of non-fullerene acceptor core structure on photophysics and device performance – We recently demonstrated that the acceptor core structure is critical for achieving high open-circuit voltages (Voc) combined with high photocurrents (Jsc) (M. Alamoudi et al., ACS Energy Letters 2018, 3, 802-811​). The acceptor core structure and acceptor’s energy levels have an impact on the energy of interfacial charge-transfer states (determining the Voc) and the fraction of charges undergoing geminate recombination (governing the Jsc). In this study, we included only two different core structures (cyclopentadithiophene (CDT) vs. indacenodithiophene (IDTT)). Further experiments are ongoing on novel nonfullerene acceptors, for instance IDT-based acceptors. Those have recently demonstrated high efficiencies combined with improved device stability  (D. Baran et al., Nat. Mater. 201716, 363).   



A precise spectroscopic picture of the loss processes in novel polymer:fullerene bulk heterojunction systems – We recently studied the impact of polymer backbone fluorination and thin film processing conditions on the photophysics of copolymer:fullerene blends (J.Gorenflot et al., Adv. Energy Mater. 2017, DOI: 10.1002/aenm.201701678). The highlight of this study: we managed to quantify the exciton and charge carrier concentrations in the blend at any point in time after photoexcitation by combining data from all-optical pump-probe spectroscopy and electro-optical time-delayed charge carrier collection field (TDCF) experimentson devices.Thereby, we could not only identify, but also quantify the different loss channels that limit the performance. We found the product of the efficiencies of the individual steps of the photocurrent generation process reproduced the device’s external quantum efficiency,​     
which strongly supports our methodological approach and data analysis. Similar studies on novel material systems developed at KSC, for instance on nonfullerene acceptor (NFA) blends, are ongoing.​​

Effect of molecular structure on charge generation,​ recombination, and device efficiency – In a joint effort with the group of Professor Pierre Beaujuge we studied the effect of linear vs. branched alkyl-sidechain substitution in PBDTTPD:PCBM bulk heterojunction solar cells on the photophysical and photovoltaic performance (C. Dyer-Smith et al., Adv. Energy Mater. 20155, 1401778). Interestingly, an all linear-substituted polymer exhibited significantly lower photovoltaic performance (~4 %) compared to a polymer substituted with a combination of linear and short-branched​ alkyl-chains (~8 %). Using ps-µs transient absorption spectroscopy and multivariate curve resolution data analysis showed: geminate recombination of charge-transfer states limits the short circuit current and increases non-geminate recombination of free charges, in turn limiting the fill factor and open circuit voltage of the devices.​​

Further recent key publications in this area are:
Adv. Sci. 2018, 10.1002/advs.201700980; Adv. Energy Mater. 2018, accepted; Adv. Energy Mater. 2017, 7 (15), accepted; Adv. Energy Mater. 2017, accepted; Nat. Commun. 2016, 7, 11585; Chem. Mater. 2016, 28 (7), 2200-2208; Nat. Commun. 2015, 6, 8778; J. Phys. Chem. C 2015, 119, 13509-13515; Macromol. Rap. Comm. 2015, 36, 1122−1128; Macromol. Rap. Comm. 2015, 36, 1054-1060; Energy Environ. Sci. 2015, 8, 1511-1522; J. Mater. Chem. A 2015, 3, 1530-1539

Metal Halide Perovskites​:

Direct observation of hole extraction from perovskites to polymeric hole transport layers – In collaboration with the groups of Stefaan De Wolf and Iain McCulloch, we recently showed, that using ultrafast transient absorption (TA) spectroscopy, we can directly monitor hole extraction from perovskite to polymeric hole transport layers (HTL) as well as we can probe carrier recombination at the perovskite/HTL interface (E. Ugur et al., J.Phys. Chem. Lett. 2019, DOI: 10.1021/acs.jpclett.9b02502). More precisely, we used the polymer PDPP-3T as HTL, since its ground state absorption is red-shifted with respect to the perovskite’s photobleach. We varied the perovskite layer thickness and thereby distinguished between carrier diffusion and extraction at the perovskite/PDPP-3T interface. Furthermore, we determined the carrier mobility by fitting the perovskite’s photobleach dynamics using a 1D diffusion model. From the PDPP-3T carrier dynamics, we deduced that hole extraction is one-order faster than recom​​​​bination of holes at the interface.

​High quality, pin-hole free, and reproducible perovskite thin films by enhanced morphology control – In collaboration with the Amassian group, we recently reported that the grain size and vertical uniformity of polycrystalline methyl-ammon​ium lead iodide (MAPbI3) perovskite thin films can be​ greatly enhanced, if glycol ethers are added to the preparation protocol of the perovskite layer (E. Ugur et al., ACS Energy Lett. 2017, DOI: 10.1021/acsenergylett.7b00526). In turn, the device efficiency increased and the reproducibility was greatly enhanced. Using in-situ UV-Vis absorption and GIWAXS experiments, we unraveled the mechanism behind an improved morphology and optimized crystal growth in perovskite polycrystalline thin films.  
Using this protocol, we are now able to make perovskite films that consistently show virtually the same spectroscopic characteristics, unlike previously obtained from conventional preparation protocols. We are now extending our studies to other metal halide perovskites to investigate device efficiency, charge recombination processes (carrier lifetime), and stability of perovskite thin films, and in particular to study the largely unknown effect of intrinsic and extrinsic defects on the photophysics and device perf​​ormance. 

Further recent key publications in this area are:
J. Phys. Chem. C 201721, 11201-11206; Nat. Mater. 201716 (2), 258-263; ACS Energy Letters 20161 (5), 1049-1056; J. Phys. Chem. C 2016120 (10), 5724-5731.

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